Such a lead accumulator with a thixotropic gel for an electrolyte, has for example been described in Swiss Patent Specification 391 807. Treated in this specification is a position-independent accumulator of limited auto-discharge capability possessing a fixed electrolyte, which remains gas tight during operation, characterized in that for the purpose of solidification, substances capable of forming a thixotropic gel are added to the electrolyte, whereby in order to facilitate pouring, the thixotropy of the gel permits the temporary liquefaction of the electrolyte. It is assumed that at the moment of filling, the particles of gel former in the zone concerned, in which there is no re-formed gel, are already associated to such an extent that the particles in direct contact with the active material are no longer capable of penetrating its narrow pores.
The lead accumulators described in CH-PS 391 807, however, have the disadvantage that only relatively small electrodes, small particularly with respect to surface area, can be employed. The reason for this situation is that the liquified form of the thixotropic gel begins at the moment of filling to revert to a solid gel. The resulting steady increase in viscosity increasingly hinders the flow of electrolyte into the less accessible regions of the cells, the result of which being that the cell completely stops functioning. For this reason, the capacity of these accumulators has been limited to approx. 20 ampere-hours.
Another disadvantage of the accumulators described in CH-PS 391 807 is that the introduced thixotropic gel, in the manner of a liquid electrolyte, first completely covers the active materials of both electrodes, whereafter it enters the pore entrances in the active materials. What results is a loss of performance at both electrodes. With regard to the negative electrodes, the consumption of oxygen produced at the positive electrode that is so important for the maintenance-free operation of long life batteries is first entirely prevented at the negative electrode. The three phase boundary system, i.e. gasiform, liquid and solid, can begin to develop only after the gel has fissured sufficiently to occasion considerable water loss. Because the gel is extremely difficult to extract from the pore channels, as a result of which the entire surface of the negative mass cannot be covered, oxygen consumption remains limited.
In German Patent Specification 1 671 693, is described another lead accumulator with a thixotropic gel as an electrolyte, which because of the employment of antimony-free alloys for the mass support of the electrodes also permits maintenance-free operation. Moreover, due to the simultaneous employment of silicic and phosphoric acids in the electrolyte, high cycle integrity is attained, which is even greater than that found in a comparable lead accumulator featuring antimony-filled mass supports. In the production of this accumulator, dry, charged plates are installed in the cells. Before filling, the cell containers are emptied. The thixotropic electrolyte rendered temporarily liquid for the purpose of filling is poured in under a vaccuum, whereafter the cell containers are again aerated.
Compared with the extraordinary extension to the lifetime of the battery granted by the simultaneous employment of silicic and phosphoric acids, especially during the unload/load cycle, the filling process that is speeded up through evacuation and aeration does not offer any special advantages with respect to the sizes of electrodes that may successfully be employed. With regard to loss of performance at both electrodes and the oxygen consumption at the negative electrode, there is only further deterioration.
In DE-PS 30 41 953, a lead accumulator is described, that besides featuring a thixotropic gel as an electrolyte, permits the installation of electrodes of any size. The delimiting of the flow paths for the electrolytes is avoided inasmuch as, for the filling of the cells, an electrolyte is used, that by reason of its composition neither represents a thixotropic gel nor can form one. The filling electrolyte contains however all the gel former required to develope a sufficiently solid gel. It likewise contains sulphuric acid, but in such a weak concentration that independent gel development can occur neither during filling nor thereafter.
The sulphuric acid required for gel formation is electrochemically bound in the active material of the electrodes. Formation of the thixotropic gel can take place only by means of a charging procedure that frees the bound sulphuric acid.
The filling electrolyte according to DE-PS 30 41 953 possesses exceptional flow properties. Its viscosity is similar to that of a gel-free electrolyte, since the discrete particles of the gel former are only slightly associated, or not at all. This, however, has the disadvantage that the gel former is able to penetrate deeper into the pores of the active material than is usually the case with a temporarily liquefied thixotropic gel, the result of which is, necessarily, a loss of efficiency at the affected electrodes. There is furthermore the disadvantage that due to direct contact with the thixotropic gel, oxygen consumption at the negative electrode, a factor so important for maintenance-free cell operation, is at first completely disrupted; it later returns, but with difficulty.